The present invention relates to a hydraulic brake-actuating device, in particular for vehicles with an antiskid control unit, comprising a pressure fluid source and a valve arrangement to be controlled by the brake pedal, the pressure fluid being fed via a regulating valve arrangement to a control chamber, which is bounded by a pressure piston acting on a master cylinder piston, with the pressure piston being adapted to be operated by the brake pedal without pressure fluid being supplied by the pressure fluid source.
With regard to such brake-actuating devices the experts are confronted with the problem of providing an emergency device for a brake unit actuated hydraulically which permits applying the brakes upon failure of the hydraulic actuating circuit.
A brake-actuating system of the aforementioned type is known from German Patent DE-OS No. 2,307,817. The valve arrangement includes two pistons, the first one of the two pistons sliding in a bore of the second one of the two pistons. The second piston is in mechanical connection with the pressure piston of a hydraulic actuating device. The pressure piston is mechanically connected with the master cylinder piston. When the controlled pressure fluid is introduced, the pressure piston will lift from the second piston of the valve arrangement and actuate the master cylinder. Due to this, the controlled pressure actuating the pressure piston will also act on the second piston of the valve arrangement. With the application of the maximum auxiliary force, the pressure piston will be spaced a considerable distance from the mechanical abutment with the second piston so that upon failure of the pressure in the hydraulic pressure fluid circuit the first piston has to be urged into mechanical abutment with the second piston of the valve arrangement for the initial braking to be continued. This requires a considerable brake pedal travel, thus, causing the positioning of the brake pedal to be adjusted to this type of emergency. Consequently, considerable structural space is wasted in the interior of the vehicle due to the location of the brake pedal, since the other actuating elements of the vehicle have to be arranged on the level of the brake pedal, too. In addition, an unfavorable sitting position results for the driver in case of very long brake pedal travels.
The lost travel until the master cylinder piston is mechanically actuated is particularly prejudicial to the so-called dual-circuit cylinder arrangements. Both master cylinders have to be mechanically actuated by the brake pedal, when the pressure in the hydraulic pressure fluid circuit fails. If it is the case that the pressure pistons of the two master cylinder arrangements are differently positioned, dependent on the pressure control of an antiskid control unit, differing lost travels will have to be overcome until both pressure pistons mechanically abut when the pressure fails in the hydraulic pressure circuit. This may have a result that with the pressure piston being unfavorably positioned only one master cylinder builds up the necessary pressure in case of an emergency braking, whereas the pressure build-up in the second master cylinder ensues only with a deceleration. A considerably longer braking operation results from this situation. The emergency braking may furthermore be endangered by a canting of the mechanical actuating device. Upon a normal mechanical actuation, the second piston and the actuating rod are in force balance with the two pressure pistons when a force is transmitted. If only one pressure piston is abutted, a torque is exerted on the piston arrangement of the valve arrangement, which leads to a blocking of the mechanical actuating device. An emergency braking is impossible in this situation.